is it 312 ?

Yes

thx

And what is the answer for n = 20. Is it 19044?

Simple backtrack can easily yield the answers up to 30 (and more). However, this didn't help me much.

Here there are if it will help you with something.

Answer for n = 6 is 12
Answer for n = 7 is 46
Answer for n = 8 is 144
Answer for n = 9 is 110
Answer for n = 10 is 312
Answer for n = 11 is 290
Answer for n = 12 is 670
Answer for n = 13 is 706
Answer for n = 14 is 1538
Answer for n = 15 is 1732
Answer for n = 16 is 3504
Answer for n = 17 is 4288
Answer for n = 18 is 8098
Answer for n = 19 is 10568
Answer for n = 20 is 19044
Answer for n = 21 is 26042
Answer for n = 22 is 45222
Answer for n = 23 is 64220
Answer for n = 24 is 108382
Answer for n = 25 is 158324
Answer for n = 26 is 261754
Answer for n = 27 is 390314
Answer for n = 28 is 635666
Answer for n = 29 is 962282
Answer for n = 30 is 1550244
Answer for n = 31 is 2372372
Answer for n = 32 is 3792560

Thank you! These answers helped me a lot.